The present invention is directed to air ion generators and, more specifically, to an apparatus for sensing and monitoring alternating current in a power supply of an air ionizer.
Controlling static charge is an important issue in continuous web operations (product moved in a continuous or nearly continuous feed) and in semiconductor manufacturing. Undesirable Triboelectric (static caused by friction) charges are introduced onto the web during handling by rollers, cutters and the like. In web operations, such undesirable charges can attract unwanted particulate matter onto the product, can cause difficult handling issues with the product, and may even cause discharges which are potentially harmful to the electronic controls that operate the machines. In semiconductor manufacturing, device defects caused by electrostatically attracted foreign matter and electrostatic discharge events contribute greatly to overall manufacturing losses.
Air ionization is an effective method of eliminating static charges on non-conductive materials and isolated conductors. Air ionizers generate large quantities of positive and negative ions in the surrounding atmosphere which serve as mobile carriers of charge in the air. As ions flow through the air, they are attracted to oppositely charged particles and surfaces. Neutralization of electrostatically charged surfaces can be rapidly achieved through this process.
Air ionization may be performed using electrical ionizers which generate ions in a process known as corona discharge. Electrical ionizers have electrodes and generate air ions through this process by intensifying an electric field around a sharp point of each electrode until it overcomes the dielectric strength of the surrounding air. Negative corona occurs when electrons are flowing from the electrode into the surrounding air. Positive corona occurs as a result of the flow of electrons from the air molecules into the electrode.
Ionizer devices take many forms such as ionizing bars, air ionization blowers, air ionization nozzles, and the like, and are utilized to neutralize static electrical charge by emitting positive and negative ions into the workspace or onto the surface of an area carrying undesirable static charges. Ionizing bars are typically used in continuous web operations such as paper printing, polymeric sheet material, or plastic bag fabrication. Air ionization blowers and nozzles are typically used in workspaces for assembling electronics equipment such as hard disk drives, integrated circuits, and the like, that are sensitive to electrostatic discharge (ESD).
To achieve the maximum possible reduction in static charges from an ionizer of a given output, the ionizer must produce amounts of positive and negative ions in order to compensate for the net charge on the web or in the workspace. That is, the output of the ionizer must increase or decrease the output of positive and/or negative ions in order to achieve a neutralized net charge on the web or in the workspace.
One prior art method of generating ions is by use of an alternating current (AC) voltage generator connected to ionizing pins (i.e., electrodes) which produces ions of one polarity for approximately 35% of a half-cycle and then, after a delay, produces ions of the other polarity for approximately 35% of a half-cycle. The positive ions and negative ions are output based upon the cycle or frequency of the AC voltage waveform and are not controlled based upon feedback of the actual charge on the web or in the workspace or on the demand for ions of a particular polarity. Such prior art devices are discussed in U.S. Pat. No. 3,936,698 (Meyer) and U.S. Pat. No. 3,714,531 (Takahashi).
A drawback to AC ionizers is the ability to monitor the actual current flow being delivered to the ionizing pins. Shields and ground references serve as an additional load on the AC high voltage, thereby drawing current that is orders of magnitude larger than the actual ion current produced by the ionizing pins. For example, FIG. 5 is a simplified electrical schematic of a prior art monitor circuit 50 that measures return current to a secondary 54 of a high voltage transformer 52 through a grounded resistor RG. When a shielded high voltage cable 51 is used to distribute AC high voltage to ionizing pins of an ionizer bar, the resulting signal across the ground resistor RG will predominantly represent a load current of the cable 51. Ion current is only a small fraction of a total current returning to the transformer and is difficult to detect relative to the large current supplied to the shield and ground references. The typical method to measure the current in the ionization system would include measuring the voltage across the ground resistor RG. Using Ohms law, I=E/R, the current can only be calculated as the total current returning to the high voltage transformer 52.
It is desirable to provide a way of accurately measuring alternating current in an ionization system. Moreover, it is desirable to provide an apparatus and method for measuring alternating current flow due to ion generation out of electrodes of an ionization system.